1. Field of the Invention
The invention concerns a wedge drive or cotter key with a first part which can be provided with a machining tool and a second part, wherein the two parts are arranged movably relative to each other, and there is provided at least one positive-action return device which engages or can engage both parts, and a third part which is connected to the first part.
2. State of the Art
Wedge drives are used in particular in the automobile industry for converting a perpendicular pressing force into a horizontal movement. In particular in the production of bodywork parts, it is possible in that way to carry out shaping processes or operations for cutting or perforating bodywork parts, which is not possible by means of a perpendicular working movement, that is to say the normal direction of movement of a press. Wedge drives must therefore be so designed that they convert very high working pressures of a press into the desired working direction, that is to say for example a horizontal direction, in which case at the same time a linear guide is provided. The pressures occurring in that case can rapidly exceed 5,000 kN. In that respect, it is also possible to arrange in a press tool a plurality of and in particular ten or more such wedge drives which perform different functions and which for that purpose operate with different angles of inclination with respect to the working direction.
In a wedge drive, a linear guide is always provided in the form of the wedge drive bed which, depending on the respective design configuration involved, is intended to provide pressing pressures of more than 100 kN with a guide play of a maximum of 0.02 mm in the respectively desired direction in accurate repetition relationship.
A drive wedge, hereinafter referred to as the driver element, is intended in that case to apply the perpendicular pressing force to the actually movable wedge drive element, the wedge drive slide, referred to hereinafter as the slider element. The slider element receives the tools required for the machining operation and therefore performs the actual machining process and is reciprocated in a driven mode in the linear guide of the press. The tools which can be mounted to a slider element for cutting or shaping a workpiece such as a bodywork part can be of different designs. In that respect it is possible to mount only for example one single perforating punch or a number of perforating punches or other tools such as for example also a number of individual blades of a total length of more than a meter. The same also applies to the post-shaping area, in which case simple shaping punches or also reshaping jaws for subsequent reshaping of various portions of a workpiece which can extend over one or more meters can be used as the tools. Therefore, in order to meet those differing requirements of non-cutting shaping in a pressing tool, wedge drives of different sizes and with a different working angle are available on the market. Examples of such wedge drives are also described in WO 02/30659 A1, WO 99/28117 and EP 0 484 588 A1.
The design configuration of the wedge drive depends on the activities to be performed, that is to say for example it is dependent on the sheet metal thickness and the sheet metal quality of the workpiece to be worked, the respective working length and the nature of the machining operation, for example cutting or shaping. In accordance with the requirements of the automobile industry, it is necessary to ensure that a wedge drive must attain at least 1,000,000 strokes with the required working force and an operational play which ensures that the respective perforating punch meets a corresponding cutting bush or counterpart die in accurately targeted relationship. A displaced encounter of a perforating punch or a cutting blade has the result that increased abrasive wear can occur at the perforating punch or cutting blade and at the cutting bushes, which in the worst-case scenario leads to fracture of the cutting or shaping tool in the form of a perforating punch, cutting blade etc. A force acts on the cutting and shaping tools not only in the actual working stroke movement for penetrating or shaping a workpiece, but also in the return movement thereof. It is precisely in the case of a perforating punch which only in regard to a certain extent cuts through a workpiece in the form of a metal sheet, and only pushes through the remainder by means of a tearing movement, a clamping action can occur in the return movement, and that clamping action in the worst-case scenario can lead to damage to the workpiece and the perforating punch or cutting blade. That effect is increased by deposits of zinc or aluminum when working with zinc or aluminum sheet which nowadays is increasingly being used in the automobile industry. Those deposits on the cutting means lead to lubrication or to the formation of an obstructive lubricant film which hinders further processing of workpieces with a correspondingly damaged cutting tool. The stripping-off force which acts on the cutting tool in the form of a perforating punch, cutting blade and so forth, when it is retracted from a workpiece, is between about 5 and 12% of the actual working force.
In a wedge drive, such a stripping-off force which is also referred to as the retraction force is applied for example by means of a return spring. It will be noted however that it has been found that such springs can apply the required stripping-off force or retraction force of between 5 and 12% of the working force, only in the rarest cases, as the structural space which is only limitedly available in a wedge drive means that it is possible to use only very small and thus weak springs. The wish on the part of the automobile industry to nonetheless maintain those values cannot be met with the spring systems available on the market such as for example coil springs, rubber or plastic springs, gas pressure springs and so forth, in particular by virtue of the small structural space available within the wedge drives. By way of example, in the case of a wedge drive involving a working force of 5,000 kN, a stripping-off or retraction force of 600 kN or more would have to be maintained, but the available spring systems only make it possible to maintain values of not even 300 kN. The result of this is that extensive and costly special solutions have to be used to maintain the required values. A further disadvantage of springs is in particular also that, with an increasing loading, they lose in terms of service life. The required values of 1,000,000 strokes cannot therefore be even approximately achieved without expensive replacement of the spring systems being required. As a result operation of a wedge drive is not only additionally increased in cost but it also leads to process uncertainty as the failure or the at least restricted operation of such a spring system cannot be estimated in advance. A failure of one such return spring has the result that the wedge drive no longer slides back into its end position and thus the machined workpiece is no longer freed for removal. That results in considerable losses and thus immense additional costs which however obviously have to be avoided. The requirement therefore is admittedly to make the retraction forces on the one hand as high as possible, while however at the same time increasing the service lives of such a wedge drive and designing with a greater degree of process certainty.
To achieve this, clamp-like positive-action return devices are known, as are used for example in the above-mentioned publications in the state of the art. Those clamp-like positive-action return devices are mounted in positively locking relationship to the wedge drive and hold the slider element and the driver element together in such a way that retraction into the end position takes place in reproducible fashion. The positive-action return devices in the state of the art however are not designed for permanent ongoing operation but only serve to release a brief sticking effect. It has further been found that operation with a particularly long manufacturing interval is also not possible with such positive-action return devices in the state of the art, in which respect one problem is that an overloaded positive-action return device breaks off and causes even greater damage, in the form of a foreign body, in the wedge drive or the press, than a wedge drive which unintentionally sticks because of a yielding spring.